It is well known that for preparing articles of acrylic polymers, for instance polymethylmethacrylate (PMMA), with high thickness, low molding temperatures are generally recommended for the reasons listed hereinafter:
articles are obtained with higher optical purity, as the degradation of the material is minimized and therefore the yellowing of the same is minimized; PA1 cooling times in the mold, normally rather long for pieces having a high thickness, are reduced; PA1 eddies, shrinkages and cooling stresses are reduced, because of the lower thermal gradient between melted mass and mold.
The typical molding temperatures normally used for PMMA being in fact of the order of 230.degree.-250.degree. C., and do not allow to obtain articles of high purity, as stressed by the yellow index, even though they show a good aesthetic aspect.
The drawback of molding processes employing low temperatures consists in that strong frictions are generated in the feeding zone with localized overheating causing in a stochastic way on the finished piece white or silver stripes which irreparably compromise the article aesthetics.
In order to overcome this drawback it is known that frictions can be reduced using external lubricants which are added on the granules surface. However the presence of external lubricants, when relatively low molding temperatures are used, causes the appearance of haze in the polymer which is well visible in articles with high thickness. The onset of this diffused haze on the whole piece leads to an article having poor aesthetic characteristics.